1. Field of the Invention
This invention relates generally to spark plug electrodes, particular to materials of the electrodes, and methods of forming the same.
2. Related Art
Spark plugs are widely used to initiate combustion in an internal combustion engine. Spark plugs typically include a ceramic insulator, a conductive shell surrounding the ceramic insulator, a center electrode disposed in the ceramic insulator, and a ground electrode operatively attached to the conductive shell. The electrodes each have a spark surface located proximate one another and defining a spark gap therebetween. Such spark plugs ignite gases in an engine cylinder by emitting an electrical spark jumping the spark gap between the center electrode and ground electrode, the ignition of which creates a power stroke in the engine. Due to the nature of internal combustion engines, spark plugs operate in an extreme environment of high temperature and various corrosive combustion gases and therefore should be fabricated of appropriate materials. When the electrodes are not fabricated of appropriate materials, the extreme working conditions may gradually increase the width of the spark gap between the center electrode and ground electrode, and may induce the misfire of spark plugs and cause subsequent loss of engine power and performance.
Spark plug electrodes often include a core formed of copper and a clad formed of at least one metal, such as a nickel alloy or at least one other metal having a coefficient of thermal expansion significantly lower than copper. The copper provides a high thermal conductivity and thus reduces the operating temperature of the electrode. The nickel alloys and other metals used to form the clad have good erosion and corrosion resistance. An example of an existing electrode includes a core formed of 100 wt % copper and a clad formed of a nickel alloy including 14.5-15.5 wt % chromium, 7.0-8.0 wt % iron, 0.2-0.5 wt % manganese, and 0.2-0.5 wt % silicon, and a balance of nickel.
The existing electrodes including a copper core and metal clad experience large temperature gradients when the engine runs between full throttle and idle operation. Oftentimes undesirable swelling, thermal mechanical stresses, and induced creep deformation occur because the copper core has a coefficient of thermal expansion significantly greater than the metal clad. The difference in coefficient of thermal expansion between the core and the clad is typically 4×10−6/K.
FIGS. 10 and 11 show how a center electrode may deform during operation. When the temperate of the center electrode increases from room temperature to operating temperature, which is typically greater than 500° C., a compressive thermal stress builds up on the copper core because the coefficient of thermal expansion of copper is significantly greater than the coefficient of thermal expansion of the metal material of the clad. The copper core may undergo a time dependent creep deformation under the compressive axial stress. The creep deformation causes the copper core to shrink axially and expand radially.
The creep deformation of the copper core also causes the clad to compress. The clad has a geometrical constraint on the deformation of the copper core and thus expands radially from the solid line to the phantom line shown in FIG. 10. The radial expansion of the clad under this stress is also a creep process. This expansion causes a tension stress along the azimuthal direction and may cause cracks in the clad or in the surrounding insulator, as shown in FIGS. 10 and 11. The thermal stresses and associated axial shrinking and radial expansion repeats each time the engine runs, which may reduce the strength and performance of the center electrode. The thermal stresses and creep dependent deformation may also occur in the ground electrode, causing the spark surface of the ground electrode to shift away from the center electrode and the spark gap to increase.